Gel coats are widely used in numerous applications as the external surface layer of composite molded articles. Gel coats are typically found on composite articles that are exposed to the environment and must exhibit moisture resistance, resistance to cracking and similar properties, or on composite articles that require a strong, flexible, abrasion and impact-resistant surface and/or a smooth glossy finish. Examples of such articles include boat hulls, bath tub enclosures, pools, spas, and body panels on cars and trucks, among others.
Such gel-coated articles are typically formed by spraying a gel coat composition onto the inside surface of an open mold, applying a fibrous composite material and a laminating resin onto the gel coat, curing the gel coat and laminating resin, then removing the cured, gel-coated article from the mold. Gel-coated articles can also be fabricated by applying the composite materials into a multi-part mold, injecting or applying the gel coat composition, closing the mold, curing the gel coat and then removing the cured, gel-coated article from the mold.
Gel coats for composite articles are typically formulated from a thermosetting base resin system such as unsaturated polyester, acrylate or unsaturated urethane type resin with incorporated fillers, pigments, thixotropic agents, and other additives. The gel coat composition should exhibit low viscosity at high shear to allow for ease of application to the mold, but also resist sagging or running after it is applied. Another important property of gel coats is surface tackiness and cure time. A gel coat desirably has a gel time of 10 to 20 minutes.
It is known to incorporate inorganic and/or organic fillers into gel coat compositions. Organic fillers for incorporation into gel coats have been prepared by two methods. In Bauchet et al, U.S. Pat. No. 8,906,502, organic filler material is cured in large blocks, then ground to the desired particle size. This approach is undesirable because the grinding of cured (cross-linked) thermosetting materials is difficult and costly.
Organic fillers in fine particle form have also been prepared by polymerization of organic resins in aqueous dispersions by Masawaki et al, US 2003/0114622. However, organic particles produced by Masawaki's method are opaque and light colored. Consequently, when incorporated as a filler into a gel coat composition, the particles contribute whiteness and opacity to the gel coat. For example, a black-pigmented gel coat composition incorporating a filler of organic particles produced by the Masawaki process can produce a gel coat that is undesirably grey in color rather than black, with visible white particles dispersed throughout. In gel coats and other applications, it is desirable that a filler impart no color or opacity that can alter the intended color of the material.